OFDM is the radio access technology selected for a number of modern wireless communications systems, e.g., WiFi, 3GPP LTE, WiMax, and the like. The main idea of OFDM is to transmit a number of narrow-band symbols in parallel in the frequency domain, which are efficiently converted to and from the corresponding time-domain waveform using Inverse Fast Fourier Transform and Fast Fourier Transform (IFFT/FFT) operations. Since each individual sub-carrier is narrow-band, the symbols are also relatively robust against frequency-selective fading, or dispersion (also known as multipath interference) in the propagation channel. For moderate multipath, even if the overall signal frequency band exhibits fading dips, the channel remains approximately flat for each symbol over its narrow frequency band, limiting the inter-symbol interference (ISI) to sufficiently low levels for acceptable results.
In order to improve the multipath robustness further, a cyclic prefix (CP) may be introduced, by prepending a copy of the last part of the time-domain OFDM symbol to the beginning of that symbol prior to transmission. Using the CP eliminates the ISI altogether, if the channel dispersion is shorter than the length of the CP, as well known in the art.
A baseline OFDM receiver structure 10 is depicted in FIG. 4. OFDM signals are received at one or more antennas 12, and processed by a front end receiver circuit 14, which may include low-noise amplification, frequency down-conversion, analog filtering, and the like. The signal is digitized by an analog-to-digital converter (ADC) 16, and baseband filtered by a digital filter 18. A timing reference is established, for example using the CP properties or known synchronization signals, and provided to a Fast Fourier Transform (FFT) processor 20. A length-N sample sequence, starting at the timing reference, is then processed by the FFT 20. The individual carriers are de-rotated to undo the per-carrier channel impact in channel estimation block 22, and the transmitted symbols are recovered by symbol detector 24.
When OFDM transmission is attempted over heavily dispersive channels, the resulting ISI may not be negligible, despite the degree of robustness built into the OFDM scheme. This result may occur even when the CP is applied. The length of the CP in a practical system is limited, since it is chosen as a compromise between providing protection in “typical” scenarios and minimizing the “wasted” transmission energy that does not directly improve the available data rates or coverage. Especially the Single Frequency Network (SFN) or Coordinated Multi-Point (CoMP) deployments, where several cell sites cooperate in transmitting the same signal, will give rise to effective multipath channels with very large delay spreads, even if each individual channel would be quite compact.
Thus, there are practical propagation scenarios where the delay spread will exceed the CP and introduce ISI in OFDM. When attempting transmission at high coding rates, even moderate ISI will be detrimental and limit the data rates experienced by the user. Simply extending the length of the CP is undesirable, as the CP is redundant information, the transmission of which consumes air interface bandwidth without contributing to the data transfer.